Transport control channel program chain linked branching

ABSTRACT

A computer program product, apparatus, and method for processing a transport control channel program with chain linked branching in an I/O processing system are provided. The method includes receiving a command message at a control unit from an I/O subsystem to perform an I/O operation. The method further includes reading a chain linked flag in the command message indicating that a subsequent command message for the I/O operation follows the command message. The method also includes reading a serialization flag in the command message requesting that device status be returned to the I/O subsystem in order to select the subsequent command message. The method additionally includes executing one or more commands in the command message, and transmitting the device status to the I/O subsystem in response to executing the one or more commands in combination with the serialization flag.

BACKGROUND

1. Field of Invention

The present disclosure relates generally to input/output (I/O)processing, and in particular, to transport control channel programchain linked branching in an I/O processing system.

2. Description of Background

Input/output (I/O) operations are used to transfer data between memoryand I/O devices of an I/O processing system. Specifically, data iswritten from memory to one or more I/O devices, and data is read fromone or more I/O devices to memory by executing I/O operations.

To facilitate processing of I/O operations, an I/O subsystem of the I/Oprocessing system is employed. The I/O subsystem is coupled to mainmemory and the I/O devices of the I/O processing system and directs theflow of information between memory and the I/O devices. One example ofan I/O subsystem is a channel subsystem. The channel subsystem useschannel paths as communications media. Each channel path includes achannel coupled to a control unit, the control unit being furthercoupled to one or more I/O devices.

The channel subsystem may employ channel command words (CCWs) totransfer data between the I/O devices and memory. A CCW specifies theI/O command to be executed. For commands initiating certain I/Ooperations, the CCW designates the memory area associated with theoperation, the action to be taken whenever a transfer to or from thearea is completed, and other options.

During I/O processing, a list of CCWs is fetched from memory by achannel. The channel parses each command from the list of CCWs andforwards a number of the commands, each command in its own entity, to acontrol unit coupled to the channel. The control unit then processes thecommands. The channel tracks the state of each command and controls whenthe next set of commands are to be sent to the control unit forprocessing. The channel ensures that each command is sent to the controlunit in its own entity. Further, the channel infers certain informationassociated with processing the response from the control unit for eachcommand. Performing I/O processing on a per CCW basis may involve alarge amount of processing overhead for the channel subsystem, as thechannels parse CCWs, track state information, and react to responsesfrom the control units.

SUMMARY

An exemplary embodiment includes a computer program product forprocessing a transport control channel program with chain linkedbranching at a control unit configured for communication with an I/Osubsystem in an I/O processing system. The computer program productincludes a tangible storage medium readable by a processing circuit andstoring instructions for execution by the processing circuit forperforming a method. The method includes receiving a command message atthe control unit from the I/O subsystem to perform an I/O operation. Themethod further includes reading a chain linked flag in the commandmessage, the chain linked flag indicating that a subsequent commandmessage for the I/O operation follows the command message. The methodalso includes reading a serialization flag in the command message, theserialization flag requesting that device status be returned to the I/Osubsystem in order to select the subsequent command message. The methodadditionally includes executing one or more commands in the commandmessage, and transmitting the device status to the I/O subsystem inresponse to executing the one or more commands in combination with theserialization flag.

Another exemplary embodiment includes an apparatus for processing atransport control channel program with chain linked branching at acontrol unit in an I/O processing system. The apparatus includes acontrol unit configured for communication with an I/O subsystem of theI/O processing system. The control unit receives a command message fromthe I/O subsystem to perform an I/O operation, and reads a chain linkedflag in the command message. The chain linked flag indicates that asubsequent command message for the I/O operation follows the commandmessage. The control unit reads a serialization flag in the commandmessage requesting that device status be returned to the I/O subsystemin order to select the subsequent command message. The control unitexecutes one or more commands in the command message, and transmits thedevice status to the I/O subsystem in response to executing the one ormore commands in combination with the serialization flag.

A further exemplary embodiment includes method for processing atransport control channel program with chain linked branching at acontrol unit configured for communication with an I/O subsystem in anI/O processing system. The method includes receiving a command messageat the control unit from the I/O subsystem to perform an I/O operation.The method further includes reading a chain linked flag in the commandmessage, the chain linked flag indicating that a subsequent commandmessage for the I/O operation follows the command message. The methodalso includes reading a serialization flag in the command message, theserialization flag requesting that device status be returned to the I/Osubsystem in order to select the subsequent command message. The methodadditionally includes executing one or more commands in the commandmessage, and transmitting the device status to the I/O subsystem inresponse to executing the one or more commands in combination with theserialization flag.

An additional exemplary embodiment includes a computer program productfor processing a transport control channel program with chain linkedbranching at a channel subsystem configured for communication with acontrol unit in an I/O processing system. The computer program productincludes a tangible storage medium readable by a processing circuit andstoring instructions for execution by the processing circuit forperforming a method. The method includes configuring a chain linked flagin a command message to indicate that a subsequent command messagefollows the command message to perform an I/O operation. The methodfurther includes configuring a serialization flag in the command messageto request that device status be returned to the channel subsystem inorder to select the subsequent command message. The method also includestransmitting the command message from the channel subsystem to thecontrol unit.

A further exemplary embodiment includes an apparatus for processing atransport control channel program with chain linked branching at achannel subsystem in an I/O processing system. The apparatus includes achannel subsystem configured for communication with a control unit ofthe I/O processing system. The channel subsystem configures a chainlinked flag in a command message to indicate that a subsequent commandmessage follows the command message to perform an I/O operation. Thechannel subsystem also configures a serialization flag in the commandmessage to request that device status be returned to the channelsubsystem in order to select the subsequent command message. Further,the channel subsystem transmits the command message from the channelsubsystem to the control unit.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 depicts one embodiment of an I/O processing system incorporatingand using one or more aspects of the present invention;

FIG. 2 depicts one embodiment of a control unit and a channel subsystem,in accordance with an aspect of the present invention;

FIG. 3 depicts one embodiment of a transport control word (TCW) channelprogram with chain linked branching, in accordance with an aspect of thepresent invention;

FIG. 4 depicts one embodiment of a link protocol used to identify acompatible control unit of an I/O processing system, in accordance withan aspect of the present invention;

FIG. 5 depicts one embodiment of a request message of the link protocolof FIG. 4;

FIG. 6 depicts one embodiment of an accept message of the link protocolof FIG. 4;

FIG. 7 depicts one embodiment of an anchor control block in accordancewith an aspect of the present invention;

FIG. 8 depicts one embodiment of a TCW in accordance with an aspect ofthe present invention;

FIG. 9 depicts one embodiment of a command message communicated from achannel subsystem to a control unit, in accordance with an aspect of thepresent invention;

FIG. 10 depicts one embodiment of a link protocol used to communicatebetween a channel and control unit to execute the TCW channel programwith chain linked branching of FIG. 3, in accordance with an aspect ofthe present invention;

FIG. 11 depicts one embodiment of a process for providing TCW channelprogram chain linked branching at a channel subsystem in accordance withan aspect of the present invention;

FIG. 12 depicts one embodiment of a process for providing TCW channelprogram chain linked branching at a control unit in accordance with anaspect of the present invention; and

FIG. 13 depicts one embodiment of an article of manufactureincorporating one or more aspects of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with an aspect of the present invention, input/output(I/O) is facilitated with chain linked branching to enable conditionalexecution of transport control channel program portions. Support forprogram controlled interrupts between portions of the transport controlchannel program may also be provided. A transport control channelprogram facilitates I/O processing by reducing communications betweencomponents of an I/O processing system used to perform the I/Oprocessing. For instance, the number of exchanges and sequences betweenan I/O communications adapter, such as a channel, and a control unit isreduced. This is accomplished through sending multiple commands and/ordata to the control unit grouped in blocks for execution at the controlunit rather than sending individual channel command words (CCWs).

Channel programs implemented with CCWs (also referred to as “CCW channelprograms”) involve a large degree of handshaking to perform tasks. Forexample, writing a 4 kilobyte block of data using a CCW channel programtypically requires an exchange to be opened, transmission of a defineextent command with data, transmission of a locate record command withdata, and transmission of a write command with data from the channel tothe control unit. The control unit typically responds by opening anexchange and sending a response to acknowledge the write command,sending a status message upon completing the write command, and closingthe exchange it opened. The channel may then respond by closing theexchange that it opened. Using a TCW channel program, a transportcommand control block (TCCB) can be sent from the channel to the controlunit as a block of commands, avoiding many of the messages between thechannel and the control unit that would otherwise be performed using aCCW channel program. For example, the TCW channel program can avoidopening an exchange to respond that the control unit received the writecommand. The cumulative effect over multiple command sequences canresult in a large time savings when running a TCW channel programinstead of a CCW channel program, and thus overall I/O processing systemthroughput is increased. In an exemplary embodiment, an I/O processingsystem can support CCW channel programs in command mode and TCW channelprograms in transport mode. Transport mode indicates that the channeltransports commands and data to the control unit without interpreting ordistinguishing between the commands and data transported.

In an exemplary embodiment, the link protocol used for command modecommunications is FICON (Fibre Connectivity). Information regardingFICON is described in “Fibre Channel Single Byte Command Code Sets-3Mapping Protocol (FC-SB-3), T11/Project 1357-D/Rev. 1.6, INCITS (March2003), which is hereby incorporated herein by reference in its entirety.The link protocol used for transport mode communications may be, forinstance, Fibre Channel Protocol (FCP). In particular, three phases ofthe FCP link protocol can be used, allowing use of host bus adaptersthat support FCP to perform data transfers. FCP and its phases aredescribed further in “Information Technology-Fibre Channel Protocol forSCSI, Third Version (FCP-3),” T10 Project 1560-D, Revision 4, Sep. 13,2005, which is hereby incorporated herein by reference in its entirety.It will be understood that other versions of these protocols and/orsimilar protocols can be used within the scope of the invention.

A plurality of commands (e.g., device command words or “DCWs”) can beincluded in a TCCB, the contents of which are located via one or moreaddress references (indirect or direct) in a transport control word(TCW). In an exemplary embodiment, the TCW is sent from an operatingsystem (OS) or other application to the I/O communications adapter,which in turn forwards the TCCB in a command message to the control unitfor processing. The control unit processes each of the commands absent atracking of status relative to those individual commands by the I/Ocommunications adapter. The plurality of commands is also referred to asa channel program, which is parsed and executed on the control unitrather than the I/O communications adapter.

A single TCCB may be constrained in size as a function of a linkprotocol or buffer size constraints, which can in turn limit the numberof commands and/or amount of data associated with the TCCB. Some I/Ooperations can include a greater number of commands or volume of datathan may be incorporated in a single TCCB. In an exemplary embodiment,chain linking of multiple TCWs with associated TCCBs is employed tocreate larger TCW channel programs, allowing a single I/O operation tospan multiple TCWs and TCCBs. A program control interrupt (PCI) is alsosupported to send an intermediate notification of progress of a chainlinked TCW channel program from the channel to the OS, enabling the OSto release or reuse resources that had previously been allocated for oneor more commands of a TCCB prior to the PCI. The PCI serves as a compactstatus indicator without requiring a full extended status message aftereach TCCB. Chain linking of a TCW channel program with PCI support canenhance conversion of a lengthy CCW channel program into a chain linkedTCW channel program that would otherwise include a larger number ofcommands than a single TCCB can hold. Furthermore, a chain linked TCWchannel program may be more efficient than running a series of separateTCW channel programs in that an extended status message can be held offuntil completion of the full chain linked TCW channel program, ratherthan sending it for each separate TCW channel program. Moreover,overhead involved in configuring and managing communications may befurther reduced when running a chain linked TCW channel program incontrast to a series of separate TCW channel programs, where eachseparate TCW channel program accomplishes a portion of an I/O operation.

Some I/O operations may be more efficient if looping or branching isemployed. For example, in order to locate data on an I/O device, asearch command with a search argument can be repeatedly executed untilthe search argument is found. In an exemplary embodiment, TCWs and TCCBsare modified to include chain linked branching support with conditionalbranching between TCCBs to perform an I/O operation. A serialize bit maybe defined in a TCW and TCCB to present jump status between DCWs. TheI/O communications adapter sends a first set of TCCBs containing “n”DCWs to the control unit. In response to the control unit presentingstatus to the I/O communications adapter for the first set of n DCWs,the I/O communications adapter examines the status and determines whichof multiple TCCBs to fetch and send to the control unit.

One example of an I/O processing system incorporating and using one ormore aspects of the present invention is described with reference toFIG. 1. I/O processing system 100 includes a host system 101, whichfurther includes for instance, a main memory 102, one or more centralprocessing units (CPUs) 104, a storage control element 106, and achannel subsystem 108. The host system 101 may be a large scalecomputing system, such as a mainframe or server. The I/O processingsystem 100 also includes one or more control units 110 and one or moreI/O devices 112, each of which is described below.

Main memory 102 stores data and programs, which can be input from I/Odevices 112. For example, the main memory 102 may include one or moreoperating systems (OSs) 103 that are executed by one or more of the CPUs104. For example, one CPU 104 can execute a Linux® operating system 103and a z/OS® operating system 103 as different virtual machine instances.The main memory 102 is directly addressable and provides for high-speedprocessing of data by the CPUs 104 and the channel subsystem 108.

CPU 104 is the controlling center of the I/O processing system 100. Itcontains sequencing and processing facilities for instruction execution,interruption action, timing functions, initial program loading, andother machine-related functions. CPU 104 is coupled to the storagecontrol element 106 via a connection 114, such as a bidirectional orunidirectional bus.

Storage control element 106 is coupled to the main memory 102 via aconnection 116, such as a bus; to CPUs 104 via connection 114; and tochannel subsystem 108 via a connection 118. Storage control element 106controls, for example, queuing and execution of requests made by CPU 104and channel subsystem 108.

In an exemplary embodiment, channel subsystem 108 provides acommunication interface between host system 101 and control units 110.Channel subsystem 108 is coupled to storage control element 106, asdescribed above, and to each of the control units 110 via a connection120, such as a serial link. Connection 120 may be implemented as anoptical link, employing single-mode or multi-mode waveguides in a FibreChannel fabric (e.g., a fibre channel network). Channel subsystem 108directs the flow of information between I/O devices 112 and main memory102. It relieves the CPUs 104 of the task of communicating directly withthe I/O devices 112 and permits data processing to proceed concurrentlywith I/O processing. The channel subsystem 108 uses one or more channelpaths 122 as the communication links in managing the flow of informationto or from I/O devices 112. As a part of the I/O processing, channelsubsystem 108 also performs the path-management functions of testing forchannel path availability, selecting an available channel path 122 andinitiating execution of the operation with the I/O devices 112.

Each channel path 122 includes a channel 124 (channels 124 are locatedwithin the channel subsystem 108, in one example, as shown in FIG. 1),one or more control units 110 and one or more connections 120. Inanother example, it is also possible to have one or more dynamicswitches (not depicted) as part of the channel path 122. A dynamicswitch is coupled to a channel 124 and a control unit 110 and providesthe capability of physically interconnecting any two links that areattached to the switch. In another example, it is also possible to havemultiple systems, and therefore multiple channel subsystems (notdepicted) attached to control unit 110.

Also located within channel subsystem 108 are subchannels (not shown).One subchannel is provided for and dedicated to each I/O device 112accessible to a program through the channel subsystem 108. A subchannel(e.g., a data structure, such as a table) provides the logicalappearance of a device to the program. Each subchannel providesinformation concerning the associated I/O device 112 and its attachmentto channel subsystem 108. The subchannel also provides informationconcerning I/O operations and other functions involving the associatedI/O device 112. The subchannel is the means by which channel subsystem108 provides information about associated I/O devices 112 to CPUs 104,which obtain this information by executing I/O instructions.

Channel subsystem 108 is coupled to one or more control units 1 10. Eachcontrol unit 110 provides logic to operate and control one or more I/Odevices 112 and adapts, through the use of common facilities, thecharacteristics of each I/O device 112 to the link interface provided bythe channel 124. The common facilities provide for the execution of I/Ooperations, indications concerning the status of the I/O device 112 andcontrol unit 110, control of the timing of data transfers over thechannel path 122 and certain levels of I/O device 112 control.

Each control unit 110 is attached via a connection 126 (e.g., a bus) toone or more I/O devices 112. I/O devices 112 receive information orstore information in main memory 102 and/or other memory. Examples ofI/O devices 112 include card readers and punches, magnetic tape units,direct access storage devices, displays, keyboards, printers, pointingdevices, teleprocessing devices, communication controllers and sensorbased equipment, to name a few.

One or more of the above components of the I/O processing system 100 arefurther described in “IBM® z/Architecture Principles of Operation,”Publication No. SA22-7832-05, 6th Edition, April 2007; U.S. Pat. No.5,461,721 entitled “System For Transferring Data Between I/O Devices AndMain Or Expanded Storage Under Dynamic Control Of Independent IndirectAddress Words (IDAWS),” Cormier et al., issued Oct. 24, 1995; and U.S.Pat. No. 5,526,484 entitled “Method And System For Pipelining TheProcessing Of Channel Command Words,” Casper et al., issued Jun. 11,1996, each of which is hereby incorporated herein by reference in itsentirety. IBM is a registered trademark of International BusinessMachines Corporation, Armonk, N.Y., USA. Other names used herein may beregistered trademarks, trademarks or product names of InternationalBusiness Machines Corporation or other companies.

Turning now to FIG. 2, one embodiment of the control unit 110 and thechannel 124 of FIG. 1 that support chain linked branching TCW channelprogram execution is depicted in greater detail. The control unit 110includes CU control logic 202 to parse and process command messagescontaining one or more TCCBs, received from the channel 124 via theconnection 120. The CU control logic 202 can extract DCWs and controldata from the TCCB(s) received at the control unit 110 to control adevice, for instance, I/O device 112 via connection 126. The CU controllogic 202 sends device commands and data to the I/O device 112, as wellas receives status information and other feedback from the I/O device112. The CU control logic 202 may use CU chain logic 204 to performvarious checks of the command messages received at the control unit 110,as well as determine an appropriate response. For example, the CU chainlogic 204 can inform the channel 124 of the maximum number of linkedcommands that are supported. The CU chain logic 204 may also handlepadding, incorrect length suppression, chain linking, and chain linkedbranching at the DCW level. While the CU chain logic 204 is depictedseparately from the CU control logic 202, it will be understood that theCU chain logic 204 can be incorporated as part of the CU control logic202.

The CU control logic 202 can access and control other elements withinthe control unit 110, such as CU timers 206 and CU registers 208. The CUtimers 206 may include multiple timer functions to track how much time asequence of I/O operations or a single I/O operation takes to complete.The CU timers 206 may further include one or more countdown timers tomonitor and abort I/O operations and commands that do not completewithin a predetermined period. In an exemplary embodiment, the CU timers206 continue to run between chained TCCBs until the chain completes asan I/O operation spanning multiple TCCBs. The CU registers 208 caninclude fixed values that provide configuration and status information,as well as dynamic status information that is updated as commands areexecuted by the CU control logic 202. The control unit 110 may furtherinclude other buffer or memory elements (not depicted) to store multiplemessages or status information associated with communications betweenthe channel 124 and the I/O device 112. The CU registers 208 may includea maximum linked commands parameter that defines the maximum number ofstreamed command messages for one I/O operation that the control unit110 supports.

The channel 124 in the channel subsystem 108 includes multiple elementsto support communication with the control unit 110. For example, thechannel 124 may include CHN control logic 210 that interfaces with CHNsubsystem timers 212 and CHN subsystem registers 214. In an exemplaryembodiment, the CHN control logic 210 controls communication between thechannel subsystem 108 and the control unit 110. The CHN control logic210 may directly interface to the CU control logic 202 via theconnection 120 to send commands and receive responses, such as transportcommand information units (TC_IUs) and response IUs. Alternatively,messaging interfaces and/or buffers (not depicted) can be placed betweenthe CHN control logic 210 and the CU control logic 202. The CHNsubsystem timers 212 may include multiple timer functions to track howmuch time a sequence of I/O operations takes to complete, in addition tothe time tracked by the control unit 110. The CHN subsystem timers 212may further include one or more countdown timers to monitor and abortcommand sequences that do not complete within a predetermined period.The CHN subsystem registers 214 can include fixed values that provideconfiguration and status information, as well as dynamic statusinformation, updated as commands are transported and responses arereceived.

In an exemplary embodiment, the channel subsystem 108 further includesCHN chain logic 216. The CHN chain logic 216 can manage chain linking,chain linked branching, and PCI generation for the channel 124. Althoughthe CHN chain logic 216 is depicted separately from the CHN controllogic 210, it will be understood that the CHN chain logic 216 can beincorporated as part of the CHN control logic 210.

FIG. 3 depicts an embodiment of a TCW channel program 300 with chainlinked branching that includes an anchor control block (ACB) 302, andTCWs 304, 306, 308, and 310. In an exemplary embodiment, the ACB 302serves as an initial control block when the first TCW in the chain (TCW304) includes two next TCW addresses for branching. In order to supportcommon formatting and sizing constraints across TCWs 304-310, the TCWfield that would otherwise be used to hold an interrogate addresslocation can hold a second next TCW address for branching. The ACB 302may be the same size as the TCWs 304-310, as a TCW format variation, andcontains the interrogate address location, which frees space in the TCW304 to support branching. The ACB 302 is chain linked to TCW 304, andTCW 304 is chain linked to both TCW 306 and TCW 310. TCW 306 is chainlinked to TCW 308. During execution of the TCW channel program 300, thechannel decides whether to branch to TCW 306 or TCW 310 based on statusreceived from the control unit 110 in response to executing DCWs in TCCB312.

The TCW 304 also includes links to the TCCB 312 and a transport statusblock (TSB) 314. The TCW 306 includes links to TCCB 316, TSB 314, anddata area 318. The TCW 308 includes links to TCCB 320, TSB 314, and dataarea 322. The TCW 310 includes links to TCCB 324, TSB 314, and data area326. The various links to TCCBs, TSBs and data areas, such as TCCB 312,316, 320, and 324, can be direct or indirect references to areas ofmemory. For example, transport blocks and data areas 312 and 316-326 canbe further subdivided into smaller blocks (contiguous or non-contiguous)and managed using indirect lists pointing to the smaller blocks (e.g.,lists of transport mode indirect data address words (TIDALs)). In anexemplary embodiment, the TCCB 312 is sent from channel subsystem 108 ofFIG. 1 to a targeted control unit 110 that parses and executes DCWs inthe TCCB 312. The control unit 110 reports conditions associated withthe execution of the DCWs in TCCB 312 to the channel 124 in a statusmessage. The CHN chain logic 216 in the channel 124 may select to sendeither TCCB 316 or TCCB 324 to the control unit 110 based on theconditions in the status message. If the channel selects TCCB 316, thenTCCB 320 is also sent to the control unit 110.

The TSB 314 may remain at the channel subsystem 108 to hold statusinformation associated with the execution of the TCCBs 312, 316, 320,and/or 324 at the control unit 110, enabling OSs 103 to access statusinformation. The data areas 318, 322, and 326 can be used to hold writedata to send to the control unit 110 or read data received from thecontrol unit 110.

In an exemplary embodiment, the TCW channel program 300 with chainlinked branching represents a single I/O operation that includesmultiple commands chained across the TCWs 304-310 and TCCBs 312, 316,320, and 324. The TCWs 304-310 each include a TSB address pointing tothe same TSB 314. If the I/O operation ends successfully, only the TSBaddress in the last TCW (TCW 308 or 310) is used by the channel 124;however, if the I/O operation ends early for whatever reason, thechannel 124 can uses the TSB address in any TCW that the channel 124 maybe working with, to obtain the memory address to store ending status inthe TSB 314.

The TCWs of FIG. 3 may also include PCI support to generate a PCI uponcompletion of commands in the associated TCCB at the control unit 110executing the TCCB. It will be understood that the configuration of andnumber of TCWs 304-310 and ACB 302 merely represents an embodiment, andis not limiting in scope, as there could be any number of TCWs chainlinked with branching, including multiple or no PCIs as part of the TCWchannel program 300. Additionally, other branching configurations can beimplemented in exemplary embodiments. For example, a TCW can branch backto its own address to loop on the same command set until a condition ismet, such as a search. Branching can also be used to skip over or loopback to any TCW in the TCW channel program 300.

In order to determine whether a control unit can support chain linkedTCW channel programs, a compatibility link protocol may be employedprior to sending chain linked TCCBs to the control unit. An example of acompatibility link protocol is depicted in FIG. 4. Channel 400 sends aprocess login (PRLI) request 404 to the control unit 402 in a defaultcommunication format. The control unit 402 responds with a PRLI accept406, which may include information defining communication parametersthat are acceptable to the control unit 402. In response to receivingthe PRLI accept 406, the channel may proceed with sending chain linkedTCCBs to the control unit 402 for execution, such as chain linked TCCBs312, 316, 320, and 324. Other messages may also be exchanged between thechannel 400 and the control unit 402 as part of link initialization andconfiguration. The channel 400 and the control unit 402 representembodiments of the channel 124 and control unit 110 of FIG. 1.

FIG. 5 depicts an example of a PRLI Request message 500, whichrepresents an embodiment of the PRLI request 404 of FIG. 4. The payloadof the PRLI Request message 500 may include a service parameter page,which includes service parameters for one or all image pairs.

The service parameter page of the PRLI Request message 500 may includemultiple fields, such as type code 502, type extension 504, maximuminitiation delay time 506, flags 508, and max linked commands 510. Eachfield in the page of the PRLI Request message 500 is assigned to aparticular byte address. Although one arrangement of fields within thepage of the PRLI Request message 500 is depicted in FIG. 5, it will beunderstood that the order of fields can be rearranged to alternateordering within the scope of the disclosure. Moreover, fields in thepage of the PRLI Request message 500 can be omitted or combined withinthe scope of the invention.

The type code field 502, located at word 0, byte 0, represents theprotocol type code, such as the Fibre Channel Single Byte Protocol typecode. For example, a value of “1B” hexadecimal in this byte indicatesthat this service parameter page 500 is defined in the selected protocol(e.g., Fiber Channel single byte). The type extension 504, located atword 0, byte 1, may further supplement the type code field 502.

The maximum initiation delay time field 506, located at word 3, byte 0,provides the maximum time (e.g., in seconds) that the channel 124 ofFIG. 1 can allow in the Initiation Delay Time field in a process Logout(PRLO) from the control unit 110.

Flags 508, in an exemplary embodiment, has the following definition:

Bit 0—Transport Mode/Command Mode. A value of this bit set to one (1)means that the sender supports both Command Mode and Transport Mode. Ifthe bit is set to zero (0), the sender only supports Command Mode. Ifthe channel 124 sets this bit to a one, then the control unit 110 mayrespond with this bit set to one if it supports Transport Mode.

Bits 1-6—Reserved.

Bit 7—First Transfer Ready for Data Disabled. If both the channel 124and control unit 110 choose to disable the first write transfer readyinformation unit (XFER_RDY IU), then the first TC_IU of all I/Ooperations performing writes between the channel 124 and control unit110 operate without using the XFER_RDY IU before the first datainformation unit (Data IU) is transmitted for the first TC_IU of an I/Ooperation. The XFER_RDY IU is transmitted to request each additionalData IU, if any for the current TC_IU and any following TC_IUs for thechannel program if any.

The max linked commands field 510 indicates the maximum count ofadditional Transport Command information units (TC_IUs) that the channel124 supports for streaming to the control unit 110 as chain linkedcommands for the same I/O device 112 after the first TC_IU has been sentto the control unit 110. Values may range from 0 to 15, with a value ofzero meaning that the channel 124 does not support chain linking ofTC_IUs. A value of X equal to one to fifteen indicates that the channel124 will send out X TC_IUs after the first TC_IU for the same I/O device112 (if there are X TCWs chain linked together) and then send out onenew TC_IU for each previous TC_IU that completed until the channelprogram is completely executed.

In one exemplary embodiment, the remaining fields in the page of thePRLI Request message 500 are reserved and/or set to zero (0). Forexample, bytes 2 and 3 of word 0, and words 1 and 2 are set to zero.Byte 1 and a portion of byte 2 of word 3 may also be reserved.

Turning now to FIG. 6, an example of a PRLI Accept message 600 isdepicted, which represents an embodiment of the PRLI accept 406 of FIG.4. The payload of the PRLI Accept message 600 may include a serviceparameter page. The service parameter page of the PRLI Accept message600 may include multiple fields, such as type code 602, type extension604, response code 606, first burst size 608, flags 610, and max linkedcommands 612. Each field in the page of the PRLI Accept message 600 isassigned to a particular byte address. Although one arrangement offields within the page of the PRLI Accept message 600 is depicted inFIG. 6, it will be understood that the order of fields can be rearrangedto alternate ordering, or can be omitted or combined, within the scopeof the disclosure.

The type code field 602, located at word 0, byte 0, is the protocol typecode, and is similar to the type code field 502 of FIG. 5. The typeextension field 604, located at word 0, byte 1, corresponds to the typeextension field 504 of FIG. 5.

The response code field 606, located at word 0, byte 2, bits 4-7, isdefined by its corresponding protocol, such as the Fibre Channel Framingand Signaling protocol (FC-FS), which is described further in “ANSIINCITS 433-2007, Information Technology Fibre Channel Link Services(FC-LS)”, July 2007, which is hereby incorporated herein by reference inits entirety.

The First Burst Size field 608, located at word 3, bytes 0-1, bits 0-15,provides the maximum amount of data (e.g., the maximum number of 4k byteblocks of data) allowed in the first Data IU that is sent immediatelyafter the first TC_IU, when the First Transfer Ready for Data Disabledflag bit (word 3, byte 3, bit 7) is set to one. A value of zero in thisfield indicates that there is no specified first burst size.

Flags 610 are similar to the flags 508 of FIG. 5 described inconjunction with the PRLI Request message 500. The control unit 110 setsvalues to these flags that correspond to the mode of operation it willrun with the channel 124.

In an exemplary embodiment, the max linked commands field 612 is themaximum count of streamed TC_IUs that the control unit 110 supports forone I/O operation. The control unit 110 responds with a count equal toor less than the value the channel 124 sent to the control unit 110 inthe service parameter page for the PRLI Request message 500. The channel124 uses the count received from the control unit 110 as the maximumnumber of linked TC_IUs queued at the control unit 110. If the controlunit 110 responds with a count of zero, this means the control unit 110does not support chain linking of TC_IUs.

In one exemplary embodiment, the remaining fields in the page of thePRLI Accept message 600 are reserved and/or set to zero (0). Forexample, bits 1-3 of word 0, byte 2, and words 1 and 2 are set to zero.Byte 3 of word 0 is reserved and set to zero. A portion of byte 2 ofword 3 may also be reserved.

An exemplary embodiment of an anchor control block (ACB) 700 is depictedin FIG. 7, as a type of TCW. The ACB 700 may be utilized by the channel124 of FIG. 1 to link to the first TCW in a chain, such as ACB 302linking to TCW 304 of FIG. 3, when the first TCW in the chain includestwo TCW address pointers. The ACB 700 is the first control block, of alist of TCWs used when multiple TCWs that branch are used by one startsubchannel command. The ACB 700 does not drive a TCCB to the controlunit 110. The channel 124 retains the address of the ACB 700 to fetchinterrogate TCW address 712 if and when the channel 124 receivesinitiative to interrogate I/O device 112.

In the exemplary ACB 700 depicted in FIG. 7, a format field 702 equal to“01” binary indicates that what follows is the ACB 700, rather than astandard TCW with a value of “00” binary. The format field 702 withvalues of “10” and “11” binary may be reserved for future TCW/ACBformats. The ACB 700 may include reserved locations 704, 706, and 708for possible future use. First TCW address field 710 is the address ofthe first TCW in a chain for execution (e.g., TCW 304). OS 103 mayconfigure the first TCW address field 710 when the ACB 700 is built. Theinterrogate-TCW address field 712 contains the address of another TCWand is used by the channel 124 to interrogate the state of an operationunder the initiative of a cancel sub-channel I/O instruction.

The ACB 700 depicted in FIG. 7 is one example of how an ACB can beconfigured. Other configurations are possible where additional fieldsare included and/or fields depicted in FIG. 7 are not included.

An exemplary embodiment of a transport control word (TCW) 800 isdepicted in FIG. 8. The TCW 800 may be utilized by the channel 124 ofFIG. 1 to set up the I/O operation and is not sent to the control unit110. The TCW depicted in FIG. 8 provides for both input and output datawithin a single I/O operation. The TCW 800 illustrates formatting thatcan be used for TCWs that employ chain linked branching, such as TCW 304of FIG. 3.

In the exemplary TCW 800 depicted in FIG. 8, a format field 802 equal to“00” binary indicates that what follows is a standard TCW 800, withother values (e.g., 01, 10, 11) equating to TCW format variations. TheTCW 800 may include reserved bits 804 for possible future use.

The TCW 800 also includes a flags field 806. Reserved flags in the flagsfield 806 may be set to zero. Examples of flags bits that are mapped tothe flags field 806 include a chain linked flag bit, a serialize flagbit, a PCI flag bit, a jump status supported flag bit, a TIDAL readflag, a TCCB TIDAL flag, and a TIDAL write flag.

When the chain linked flag bit set to a one, this informs the channel124 that the next TCW address field 828 is to be used as the next TCW tobe executed for the continuation of the I/O program. Counters, timers,and status tracking (e.g., CU timers 206 and/or CHN subsystem timers 212of FIG. 2) can continue from one TCCB to the next TCCB when the chainlinked flag is set to a one, such as between TCCBs 312 and 316 or 324.If the serialize flag bit is set to zero, exchanges may be closed by thecontrol unit 110 for the intermediate TCCBs that were executedsuccessfully with an equivalent of FCP zero status in the associatedtransport response IU. If the serialize flag bit is set to one, a statusresponse is sent in a transport response IU. A full transport responseIU with extended status is not transferred until the last TCCB of thechain linked channel program is executed or until the control unit 110encounters an early end condition. Since the TCW 800 remains local tothe channel 124, the state of the chain linked flag can be sent to thecontrol unit 110 as a chain linked TCCB flag in a TCCB as part of aTC_IU.

If the chain linked flag bit is set to a one and the serialize flag bitis set to a one, the channel 124 waits until the current TCW hascompleted before fetching the next TCW and transmitting the next TCCB tothe control unit 110 (e.g., TCW 304 to TCW 306 or 310). In an exemplaryembodiment, the serialize flag bit is set to a one if software isappending another TCW to the current TCW or if common data addressesexist for the current TCW and the following TCWs. Also, the serializeflag bit may be set to a one if the next TCW/TCCB to be executed isdependent on the ending device status from the I/O device 112. Aserialization required TCCB flag bit in a TCCB is also set to a one,when the serialize flag bit is set to one, informing the control unit110 to send the device status to the channel 124 in the transportresponse IU.

If the chain linked flag and the PCI flag are set, the channel 124generates an intermediate status interrupt when the TCW 800 iscompleted. This may result in marking the associated sub-channel asSub-channel Active, Device Active and intermediate status pending.

The jump status supported flag bit indicates whether jump status issupported for the TCW 800. The control unit 110 may send jump statusencoded as a combination of a channel end (CE), a device end (DE), and astatus modifier (SM) set in the transport response IU in response todetermining that non-sequential execution between DCWs in TCCBs isdesired. SM indicates that the control unit 110 detected that astatus-modifying condition has occurred, and that a non-sequentialinstruction should be executed, rather than continuing to the nextsequential instruction. When jump status is received, as determined bythe combination of CE, DE and SM, and the chain linked, serialize, andjump status supported flag bits are all set to a one, then words 14 and15 of TCW 800 are used to fetch the next TCW to execute (next TCWaddress for CE, DE, and SM status field 830). If the jump statussupported flag bit is set to a zero and jump status is received, thechannel 124 generates a program check.

In an exemplary embodiment, the TIDAL read flag is set to one wheninput-data address field 818 contains an address of a TIDAL. If theTIDAL read flag is set to zero, then the input-data address field 818contains a data address. In an exemplary embodiment, the TCCB TIDAL flagis set to one when TCCB address field 822 contains an address of aTIDAL. If the TCCB TIDAL flag is set to zero, then the TCCB addressfield 822 directly addresses the TCCB. The TCCB TIDAL flag allows theoperating system software or hyper-visor to layer function and prefixuser channel programs. In an exemplary embodiment, the TIDAL write flagis set to one when output-data address field 816 contains an address ofa TIDAL. If the TIDAL write flag is set to zero, then the output-dataaddress field 816 contains a data address.

The TCW 800 also includes a TCCB length field 810 which indirectlyrepresents the length of the TCCB and may be utilized to determine theactual length of the TCCB.

Read/write bits 812 in the TCW 800 are utilized to indicate whether datais being read and/or written as a result of executing the TCW 800. In anexemplary embodiment, the read bit in the read/write 812 bits is set toone to indicate that input data is being transferred from an I/O device112 to system storage (e.g., main memory 102) in the host system 101 asa result of executing the TCW 800. The write bit in the read/write bits812 is set to one to indicate that output data is being transferred fromsystem storage (e.g., main memory 102) in the host system 101 to an I/Odevice as a result of executing the TCW 800.

The output-data address field 816 includes the address for the outputdata (if any). As described previously, the contents of the output-dataaddress field 816 may be an address of a TIDAL for output data (e.g., anindirect address) or the actual address of the output data (e.g., adirect address). The input-data address field 818 includes the addressfor the input data (if any). As described previously, the contents ofthe input-data address field 818 may be an address of a TIDAL for inputdata or the actual address of the input data.

The TCW 800 also includes a transport-status-block address field 820. Aportion (e.g., the extended status part) of a completion status in atransport response IU for an I/O operation is stored at this address.The TCCB address field 822 in the TCW 800 includes an address where theTCCB is located in system storage. As described previously, the TCCB isthe control block where the DCWs to be executed for the TCW 800 reside.Also as described previously, the contents of the TCCB address field 822may be an address of a TIDAL for the TCCB or the actual address of theTCCB.

The output count field 824 in the TCW 800 indicates the amount of outputdata to be transferred by the TCW/TCCB for an output operation. In anexemplary embodiment, the output count field 824 specifies the number ofbytes in the output storage area designed by the TCW (the output-dataaddress 816) to be transferred. The input count field 826 in the TCW 800indicates the amount of input data to be transferred by the TCW/TCCB foran input operation. In an exemplary embodiment, the input count field826 specifies the number of bytes in the input storage area designed bythe TCW (the input-data address 818) to be transferred.

In an exemplary embodiment, words 12 and 13 of TCW 800 are used as anext TCW address for CE, DE status field 828, holding the address of thenext TCW to be executed when the status received from the I/O device 112is CE and DE, and the chain linked and the serialize flag bits are setto a one. For example, in the TCW channel program 300 with chain linkedbranching of FIG. 3, the next TCW address for CE, DE status is field 828of TCW 304 may be the address of TCW 306.

As previously described, words 14 and 15 of TCW 800 may be the next TCWaddress for CE, DE, and SM status and field 830 is holding the addressof the next TCW to be executed when the status received from the I/Odevice 112 is jump status (CE, DE, SM), and the chain linked, serialize,and jump status supported flag bits are set to a one. For example, inthe TCW channel program 300 with chain linked branching of FIG. 3, thenext TCW address for CE, DE, and SM status is field 830 of TCW 304 andmay be the address of TCW 310. Thus, depending upon status returned fromthe control unit 110, the CHN chain logic 216 can select between atleast two TCWs to determine the next TCW for execution. The next TCWaddress fields 828 and 830 may point to any TCW that is part of thechannel program, including looping back to the same TCW to continuouslyexecute a sequence of commands.

The TCW 800 depicted in FIG. 8 is one example of how a TCW can beconfigured. Other configurations are possible where additional fieldsare included and/or fields depicted in FIG. 8 are not included.

One example of a command message 900, e.g., a transport command IU,communicated from the channel subsystem 108 to the control unit 110 toexecute a TCW channel program is depicted in FIG. 9. The command message900 illustrates formatting that can be used for a variety of TC_IUs. Thecommand message 900 includes a header 902, a transport command header(TCH) 904, a transport command area header (TCAH) 906, a transportcommand area (TCA) 908, and a transport command area trailer (TCAT) 910.In an exemplary embodiment, the TCCBs 312, 316, 320, and 324 of FIG. 3utilize formatting as depicted in the TCAH 906, TCA 908, and TCAT 910.

The header 902 may include multiple words as address header 912,defining the highest level of header in the command message 900. Theheader 902 may include information such as channel and control unitimage IDs and a device address.

The TCH 904 includes a sequence number 913. The sequence number 913informs the control unit 110 of the order to execute multiple commandmessages 900 that are all part of the same channel I/O operationtargeting an I/O device (e.g., I/O device 112). The sequence number 913starts at (01h) in the first TC_IU for each start to the I/O device 112independent of the value it ended on for the last start to the same I/Odevice 112. If an I/O operation only contains one TCW/TCCB, then thevalue of the sequence number 913 is set to zero. The TC_IUs chain linkedtogether are executed in the order of the sequence numbers, even if theTC_IUs are received at the control unit 110 out of order.

The TCH 904 includes task information 914, which may be set to areserved value, e.g., zero, while operating in transport mode. The TCH904 also includes L1 length 916 and read/write field 918. The L1 length916 defines the length of the TCA 908 in words +1. The L1 length 916 canbe used to limit and define the size of the TCA 908. The read/writefield 918 defines whether read data, write data, or no data is beingtransferred in the command message 900, where a read is a transfer fromthe control unit 110 to the channel subsystem 108.

The TCAH 906 includes format field 920 and control field 922. The formatfield 920 and control field 922 may be set to fixed values, such as 7Fhexadecimal and zero respectively, to indicate that a variable lengthformat is used, as defined by SPC-4. SPC-4 is further described in “SCSIPrimary Commands-4 (SPC-4)”, Project T10/1731-D, Rev 11, INCITS (May2007), which is hereby incorporated herein by reference in its entirety.The TCAH 906 additionally includes reserved fields 924 and 926,TCCB-flags 927, as well as L2 length 928.

The TCCB-flags 927 inform the control unit 110 about the characteristicsof the command message 900 (the current TC_IU). The TCCB-flags 927 mayinclude a chain linked TCCB flag bit and a serialization required flagbit. The chain linked TCCB flag set to a one informs the control unit110 that there is another TC_IU following the current TC_IU that is partof the same I/O operation. Counters, timers, and status tracking (e.g.,CU timers 206 and/or CHN subsystem timers 212 of FIG. 2) can continuefrom one TCCB to the next TCCB when the chain linked TCCB flag is set toa one, and a CC bit is set to a one in the last DCW (e.g., DCW 946) inthe TCA 908 for this TC_IU. If the serialization required flag bit isnot set to a one, the exchange may be closed when the TC_IU is executedsuccessfully with an equivalent of FCP zero status, which equates tochannel end (CE), device end (DE) only status. If the serializationrequired flag bit is set to a one, then device status is sent in theresponse IU as further described herein. No extended status istransferred until the last TC_IU for the TCW channel program is executedor for the TC_IU that ended the TCW channel program. The channel 124sends the next TC_IU to the control unit 110 based on the serialize flagbit in the TCW 800 and the value of a TC_IU streaming count, which canbe tracked in the CHN subsystem registers 214 of FIG. 2. If theserialize flag is set to a zero, the channel 124 sends TC_IUs up to themax linked commands (e.g., max linked commands 612), and then sends thesubsequent TC_IUs as each previous TC_IU is completed. If the serializeflag bit is set to a one, the channel 124 waits until status is receivedfor the last TC_IU sent to the control unit 110 before sending the nextTC_IU.

The serialization required flag bit has no meaning if the chain linkedTCCB flag bit is not set to a one. The serialization required flag bitinforms the control unit 110 that even though the chain linked TCCB flagbit is set to a one, the next TC_IU will not be seen by the control unit110 until device status is sent to the channel 124 in an 8-wordtransport response IU. However, extended status is not sent in thetransport response IU for this case.

The L2 length 928 is also referred to as transport-command-area length(TCAL), and may represent the number of bytes after this position in thecommand message 900. The L2 length 928 limits the size of the TCA 908.The TCAH 906 further includes a service action code 930, reserved field932, priority 934, and reserved field 936. The service action code 930defines the type of DCWs used in the TCA 908. The priority 934 can beset equivalent to a priority byte of a FICON command header as definedin FC-SB-3.

The TCA 908 includes DCW one and control data 940, DCW two 942, DCWthree 944, and DCW four 946. The DCW one and control data 940 includesDCW fields such as a command 948, flags field 950, a reserved field 952,control data (CD) count 954, and data byte count 956. The command 948may be equivalent to a CCW command byte, but directly interpreted by thecontrol unit 110 rather than the channel subsystem 108. The flags field950 includes reserved bits as well as one or more bits assigned toparticular functions, such as indicating whether an additional DCWexists in the TCA 908 as part of a command chain. The flags field 950may also include a command chain (CC) flag bit.

The CC flag bit indicates a command chain to the next DCW in the TCA908. The CC flag bit set to zero means that the associated DCW is thelast DCW of the program. The CC flag bit can be set in the last DCW ofthe TCA 908 if the chain linked TCCB flag is set in the TCCB-flags field927 and the chain linked flag bit is set in the flags field 806 in theTCW 800.

The CD count 954 is the byte count of control data 958. The CD count 954may be padded up to the next 4-byte boundary so that subsequent DCWsstart on a 4-byte boundary. The data byte count 956 is a four-byte countof data without padding, e.g., customer data. The control data 958exists when the CD count 954 is not zero. In the exemplary commandmessage 900, the DCW two 942, DCW three 944, and DCW four 946 containsubstantially similar fields as the DCW one and control data 940. Forexample, command 960, 970, and 980 are formatted in a similar fashion asthe command 948. Furthermore, flags field 962, 972, and 982 areformatted similar to the flags field 950. Additionally, CD count 966,976, and 986 are formatted similar the CD count 954, and data byte count968, 978, and 988 are similarly formatted to the data byte count 956.Although only four DCWs, including one DCW with control data (i.e., DCWone and control data 940) are depicted in the command message 900, itwill be understood that a varying number of DCWs with and withoutcontrol data can be included in the command message 900, including asingle DCW.

The TCAT 910 includes a longitudinal redundancy check (LRC) word 990calculated on the entire command message 900. The LRC word 990 can begenerated through applying an exclusive-or operation to an initial seedvalue with each field included in the LRC calculation in succession. TheTCAT 910 also includes a transport data byte count 992 indicating thetotal number of bytes transferred for a read or write I/O operation. Ifboth the read and write bits are active in read/write field 918, thenthe transport data byte count 992 is for the write data, andbidirectional read data length 994 in TCAT 910 is the read transportdata byte count.

Unusual ending conditions may be handled as follows when multipleTCWs/TCCBs are chained to form a chain-linked TCW channel program. Forchain linked TCWs channel programs, a halt subchannel command causes allactive exchanges to be aborted for the I/O device 112 and the subchannelto be returned to the OS 103 with primary, secondary and alert status. Aclear subchannel command for chain-linked TCW channel programs may causeall active exchanges to be aborted for the I/O device 112, followed bysending a selective reset command to the I/O device 112.

For the case where the channel 124 is sending multiple TCCBs (in TC_IUs)chain linked together to the control unit 110, if the control unit 110cannot execute any one of the TCCBs, the control unit 110 can sendterminating ending status, busy status (can only be sent in response tothe first TC_IU of a channel program) or retry status, with a statusconfirm, on the exchange for the TCCB that is ending early. The controlunit 110 also closes other outstanding exchanges for the same I/Ooperation, which have a sequence number greater than the sequence numberof the exchange on which the terminating status was sent. When thechannel 124 detects a terminating ending status IU with the request fora confirm request, the channel 124 stops sending new TCCBs to thecontrol unit 110 for that operation. All other exchanges for that I/Ooperation that are not closed after a timeout period (for example, 100milliseconds) are aborted by the channel 124. When all of the otherexchanges are closed for the I/O operation, the channel 124 sends theconfirm message, which closes the final exchange.

If one of the exchanges, out of many that were opened to send TCCBschain linked together to the control unit 110 is lost, the channel 124times out that exchange and send a Read Exchange Concise (REC) to thecontrol unit 110 inquiring about the exchange. If the control unit 110informs the channel 124 that it does not have the exchange, the channel124 aborts outstanding exchanges to the control unit 110 for the I/Ooperation.

FIG. 10 depicts one embodiment of a link protocol used to communicatebetween a channel 1000 and control unit 1002 to execute the TCW channelprogram with chain linked branching of FIG. 3, where the channel 1000and control unit 1002 are embodiments of the channel 124 and controlunit 110 of FIG. 1. Once the channel 124 and the control unit 110establish that Transport Mode is supported and the maximum number oflinked commands is established, the TCW channel program with chainlinking can be executed. An OS, such as OS 103 of FIG. 1, builds the ACB302 and TCWs 304, 306, 308 and 310 associated control blocks TCCB 312,316, 320 and 324 shown in FIG. 3 and executes a start subchannel commandwith an address in an operation request block that points to ACB 302.The ACB 302 provides a pointer to the first TCW in the chain (e.g.,first TCW address field 710 pointing to TCW 304), and is also thecontrol block that the channel 1000 re-fetches to acquire theinterrogate address (e.g., interrogate-TCW address field 712) when thechannel 1000 is given initiative to perform an interrogate. Assume inthis example that the chain linked flag bit is set to a one in TCWs 304and 306 and the serialize flag bit is set to a one in TCW 304.

Upon attempting to fetch the first TCW, the channel 1000 discovers thatit is actually an ACB (ACB 302), based on format field 702. The channel1000 remembers the address of the ACB 302 and uses the first TCW addressfield 710 in the ACB 302 to fetch TCW 304 and the associated TCCB 312.

The channel 1000 sends TCCB 312 in TC_IU 1004, opening exchange A with asequence number of one to the control unit 1002. Because the serializeand chain linked flag bits are set to a one in TCW 304, the channel 1000will not fetch the next TCW until status is received from the controlunit 1002 (e.g., status of I/O device 112) for TC_IU 1004. In TCW 304and in TCCB 312 for this example, both the read and write bits are setto a zero, which tells the channel 1000 and the control unit 1002 thatno customer data will be transferred for TCW 304/TCCB 312.

At the control unit 1002, the chain linked TCCB flag bit set to a one inTC_IU 1004 informs the control unit 1002 that another TC_IU with asequence number one greater than the sequence number of the TC_IU 1004is part of this I/O operation. The serialization required flag bit isalso set to a one, informing the control unit 1002 that the next TC_IUwill not be sent by the channel 1000 until device status is sent to thechannel 1000 for TC_IU 1004.

When the control unit 1002 completes the execution of the TC_IU 1004 atI/O device 112, it sends a 32-byte Transport Response IU 1006 thatincludes the device status that also closes exchange A and informs thechannel 1000 that TC_IU 1004 has completed. For this example, assume thedevice status is CE, DE and SM (jump status), which causes the channel1000 to fetch TCW 310 of FIG. 3 using the next TCW address for CE, DE,and SM status field 830 in TCW 304.

The channel 1000 sends TCCB 324 in TC_IU 1008 opening exchange B with asequence number of 2 to the control unit 1002. The control unit executesTC_IU 1008 at the I/O device 112 and sends the data read from the I/Odevice 112, by read DCW commands in TC_IU 1008, to the channel 1000 asdata IUs 1010 on exchange B. When the control unit 1002 completes TC_IU1008, it sends a complete Transport Response IU 1012 that includes theextended status that includes 48 to 64 (or more) bytes and closesexchange B and informs the channel 1000 that the entire I/O operationhas completed. The channel 1000 presents primary status to the OS (e.g.,OS 103), informing the OS that the I/O operation has completed.

In an exemplary embodiment, extended status includes various timingparameters that can be continued between TC_IUs, such as TC_IUs 1004 and1008, as calculated using CU timers 206 of FIG. 2. For example, extendedstatus can include a total device time parameter, defer time parameter,queue time parameter, device busy time parameter, device active onlytime parameter, and appended device sense data. The total device timeparameter is the elapsed time from when the control unit 1002 receivedthe TC_IU 1004 until sending the transport response IU 1012 for the I/Ooperation. The defer time parameter indicates control unit defer time.This is the time accumulated by the control unit 1002 working with theI/O device (e.g., I/O device 112) when no communication with the channel1000 is performed. The queue time parameter is the time that an I/Ooperation is queued at the control unit 1002, but does not include queuetime for device busy time where the I/O device is reserved by anotherchannel 1000 under control of a different OS (e.g., OS 103) on the samesystem or on another system. The device busy time parameter is the timethat a TC_IU is queued at the control unit 1002 waiting on a device busycaused by the I/O device being reserved by another channel 1000 undercontrol of a different OS on the same system or on another system. Thedevice active only time parameter is the elapsed time between a CE and aDE at the control unit 1002, when the control unit 1002 holds the CEuntil DE is available. The appended device sense data is supplementalstatus that the control unit 1002 provides conditionally in response toan active unit check (UC) bit in the device status.

FIG. 11 depicts a process 1100 for providing TCW channel program chainlinked branching at a channel subsystem in accordance with an exemplaryembodiment, and is described in reference to the I/O processing system100 of FIG. 1 and subsequent figures. The process 1100 is also describedin conjunction with process 1200 as depicted in FIG. 12 for providingTCW channel program chain linked branching at a control unit, such asbetween channel 124 of channel subsystem 108 and control unit 110 ofFIG. 1. In an exemplary embodiment, the CHN chain logic 216 of FIG. 2manages processing associated with chain linked branching for thechannel 124, and the CU chain logic 204 manages processing associatedwith chain linked branching for the control unit 110. At block 1102, thechannel 124 of channel subsystem 108 configures a chain linked flag in acommand message to indicate that a subsequent command message followsthe command message to perform an I/O operation. At block 1104, thechannel 124 of channel subsystem 108 configures a serialization flag inthe command message to request that device status be returned in orderto select the subsequent command message. At block 1106, the channel 124of channel subsystem 108 transmits the command message from the channel124 of channel subsystem 108 to the control unit 110.

At block 1202, the control unit 110 receives the command message fromthe channel 124 of channel subsystem 108 to perform the I/O operation.For example, the command message may be TC_IU 1004 of FIG. 10 withformatting as depicted in FIG. 9.

At block 1204, the control unit 110 reads the chain linked TCCB flag inthe command message. The chain linked TCCB flag indicates that thesubsequent command message for the I/O operation follows the commandmessage, such as TC_IU 1008 following TC_IU 1004 of FIG. 10.

At block 1206, the control unit 110 reads the serialization flag in thecommand message. The serialization flag requests that device status bereturned to the channel 124 of channel subsystem 108 in order to selectthe subsequent command message.

At block 1208, the control unit 110 executes one or more commands in thecommand message. The one or more commands may be DCWs, such as DCWs940-946 of FIG. 9, requesting to read or write data to the I/O device112. The control unit 110 receives device status in response toexecuting the one or more commands. The device status can include CE,DE, and/or SM, where the combination of CE, DE, and SM is a jump status.

At block 1210, the control unit 110 transmits the device status to thechannel 124 of channel subsystem 108 in response to executing the one ormore commands in combination with the serialization flag. The devicestatus may be transmitted in a transport response IU without extendedstatus, such as transport response IU 1006 of FIG. 10.

Returning to FIG. 11, at block 1108, the channel 124 of channelsubsystem 108 receives the device status in response to transmitting thecommand message. At block 1110, the channel 124 of channel subsystem 108selects the subsequent command message in response to the receiveddevice status. The channel 124 examines device status received in thetransport response IU and determines which TCW to access for a TCCBholding the subsequent commands. If the device status received includesCE and DE, then the channel 124 reads the next TCW address for CE, DEstatus field 828 of the current TCW to determine the next TCW/TCCB asthe subsequent command message including a subsequent set of one or morecommands sequential to the current commands. If the device statusreceived includes CE, DE and SM, then the channel 124 reads the next TCWaddress for CE, DE, and SM status field 830 of the current TCW todetermine the next TCW/TCCB as the subsequent command message includinga subsequent set of one or more commands non-sequential to the currentcommands. For example, if CE, DE, and SM are set after executingcommands in TCCB 312, then the channel 124 can select TCW 310 and TCCB324 as the next TCW/TCCB; however, if SM is not set after executingcommands in TCCB 312, then the channel 124 can select TCW 306 and TCCB316 as the next TCW/TCCB. At block 1112, the channel 124 of channelsubsystem 108 transmits the selected subsequent command message from thechannel 124 of channel subsystem 108 to the control unit 110.

Additional subsequent messages can be received at the control unit 110as part of the chain linked channel program. The control unit 110 canread a chain linked flag (e.g., chain linked TCCB flag in TCCB-flags 927of FIG. 9) in the command message to determine whether subsequentcommand messages are expected to follow the first command message aspart of the I/O operation. In response to determining that thesubsequent command message is expected, and upon executing the one ormore commands received, the control unit 110 may continue to runcounters associated with the I/O operation to span multiple commandmessages (e.g., CU timers 206), and transmit a transport responsemessage without extended status. In similar fashion, each commandmessage received can be analyzed to determine whether additional commandmessages are expected as part of the chained I/O operation. The controlunit 110 receives the subsequent command message including a subsequentset of one or more commands, and examines command chain flags associatedwith each command (e.g., CC bits of the DCW flags 950, 962, 972, and/or982) in the subsequent one or more commands received to locate a finalchain linked command. The control unit 110 transmits a response messagewith extended status for the I/O operation in response to locating andexecuting the final chain linked command, in which case the subsequentcommand message is the final command message of the chain. Shorterstatus messages communicated as transport response messages withoutextended status can provide the channel 124 with intermediate statusafter each command message is executed prior to the final commandmessage. Upon executing the commands of the final command message,extended status is transmitted that provides additional information andstatus for the full I/O operation.

The control unit 110 can also handle other error conditions. Forexample, the control unit 110 may determine that one or more commandsassociated with a communication exchange cannot execute. The controlunit 110 can respond sending a termination status message to the channel124 of the channel subsystem 108 indicating an inability to execute. Thecontrol unit 110 closes open communication exchanges with sequencenumbers greater than the sequence number associated with the one or morenon-executable commands. For example, if the control unit 110 hasreceived sequence numbers 1, 2, 3, and 4 on exchanges A, B, C, and D,and an error occurs in executing commands associated with sequencenumber 2, the control unit 110 can notify the channel 124 of the erroron exchange B with extended status, with a request for a confirm, onexchanges B, and closes exchanges C, and D. The channel 124 will closeexchange B with a confirm on exchange B after it has seen that exchangesA, C and D have been closed (assuming A closes after successfulcompletion of sequence 1 commands).

Technical effects of exemplary embodiments include chaining of multipleTCW and TCCBs together with conditional links to form a transportcontrol channel program with chain linked branching that spans multipleTCWs and TCCBs for an I/O operation. The channel reads TCW contents forformatting constraints to determine whether a TCW is actually an ACB,and to further determine which addresses to access in response to statusreturned from the control unit. A channel may inform a control unit thata subsequent command message is chain linked to a current commandmessage and that the channel will select a specific subsequent commandmessage in response to the control unit returning device statusassociated with the current command message. If the channel receives ajump status, it can fetch a non-sequential TCW and TCCB, rather thanproceeding to the next immediate TCW and TCCB in the chain.Non-sequential TCWs/TCCBs can be a branch back to the current TCW, ajump ahead in the same chain path as would be reached sequentially or aredirection to a different chain path that would not otherwise beaccessible during sequential progression of the channel program. Thechannel may also enable periodic status interrupts to be sent while atransport control channel program is executing but not fully complete.Periodic status enables the host to confirm that a number of commandshave executed and thus buffers associated with the commands that havecompleted can be released or reused without waiting for the full programto complete.

The capabilities of the present invention can be implemented insoftware, firmware, hardware or some combination thereof

As will be appreciated by one skilled in the art, the present inventionmay be embodied as a system, method or computer program product.Accordingly, the present invention may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.” Furthermore,the present invention may take the form of a computer program productembodied in any tangible medium of expression having computer usableprogram code embodied in the medium. An example includes computerprogram product 1300 as depicted in FIG. 13 on a computer usable medium1302 with computer program code logic 1304 containing instructionsembodied in tangible media as an article of manufacture. There may bemultiple computer program products 1300, with each directed to implementfunctional processes on separate processing circuitry. For example, theprocesses 1100 and 1200 of FIGS. 11 and 12 can be embodied as computerprogram code logic 1304 on separate computer program products 1300, withone executable on the host system 101 of FIG. 1 and the other executableat one or more control units 110 of FIG. 1. Alternatively, the processes1100 and 1200 can be stored as computer executable code on a singlecomputer program product 1300.

Any combination of one or more computer usable or computer readablemedium(s) may be utilized. The computer-usable or computer-readablemedium may be, for example but not limited to, an electronic, magnetic,optical, electromagnetic, infrared, or semiconductor system, apparatus,device, or propagation medium. More specific examples (a non-exhaustivelist) of the computer-readable medium would include the following: anelectrical connection having one or more wires, a portable computerdiskette, a hard disk, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an optical fiber, a portable compact disc read-only memory(CDROM), an optical storage device, a transmission media such as thosesupporting the Internet or an intranet, or a magnetic storage device.Note that the computer-usable or computer-readable medium could even bepaper or another suitable medium upon which the program is printed, asthe program can be electronically captured, via, for instance, opticalscanning of the paper or other medium, then compiled, interpreted, orotherwise processed in a suitable manner, if necessary, and then storedin a computer memory. In the context of this document, a computer-usableor computer-readable medium may be any medium that can contain, store,communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, or device.The computer-usable medium may include a propagated data signal with thecomputer-usable program code embodied therewith, either in baseband oras part of a carrier wave. The computer usable program code may betransmitted using any appropriate medium, including but not limited towireless, wireline, optical fiber cable, RF, etc.

Computer program code for carrying out operations of the presentinvention may be written in any combination of one or more programminglanguages, including an object oriented programming language such asJava, Smalltalk, C++ or the like and conventional procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The computer program code logic 1304 of FIG. 13 represents anembodiment of program code. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

The present invention is described with reference to flowchartillustrations and/or block diagrams of methods, apparatus (systems) andcomputer program products according to embodiments of the invention. Itwill be understood that each block of the flowchart illustrations and/orblock diagrams, and combinations of blocks in the flowchartillustrations and/or block diagrams, can be implemented by computerprogram instructions. These computer program instructions may beprovided to a processor of a general purpose computer, special purposecomputer, or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the flowchartand/or block diagram block or blocks.

These computer program instructions may also be stored in acomputer-readable medium that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablemedium produce an article of manufacture including instruction meanswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide processes for implementing the functions/actsspecified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneore more other features, integers, steps, operations, elementcomponents, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A computer program product for processing a transport control channelprogram with chain linked branching at a control unit configured forcommunication with an input/output (I/O) subsystem in an I/O processingsystem, the computer program product comprising: a tangible storagemedium readable by a processing circuit and storing instructions forexecution by the processing circuit for performing a method comprising:receiving a login request message; transmitting a login accept messagein response to the login request message, the login accept messageincluding a maximum linked commands field indicating a maximum count ofadditional command messages after a first command message that thecontrol unit supports; receiving a command message at the control unitfrom the I/O subsystem to perform an I/O operation, wherein the receivedcommand message is one of the first command message and the additionalcommand messages; reading a chain linked flag in the received commandmessage, the chain linked flag indicating that a subsequent commandmessage for the I/O operation follows the received command message;reading a serialization flag in the received command message, theserialization flag requesting that device status be returned to the I/Osubsystem in order to select the subsequent command message; executingone or more commands in the received command message; and transmittingthe device status to the I/O subsystem in response to executing the oneor more commands in combination with the serialization flag.
 2. Thecomputer program product of claim 1 wherein the method furthercomprises: determining that the subsequent command message is expectedin response to the chain linked flag; and continuing to run countersassociated with the I/O operation to span the received command messageand the subsequent command message.
 3. The computer program product ofclaim 2 wherein the method further comprises: receiving the subsequentcommand message including a subsequent set of one or more commands;examining command chain flags associated with each command in thesubsequent set of one or more commands received to locate a final chainlinked command; and transmitting a response message with extended statusfor the I/O operation in response to locating and executing the finalchain linked command.
 4. The computer program product of claim 1 whereinthe device status includes a channel end (CE) and a device end (DE)instructing the I/O subsystem to send the subsequent command messagecontaining a sequential command following the one or more commands inthe received command message.
 5. The computer program product of claim 1wherein the device status includes a channel end (CE), a device end(DE), and a status modifier (SM) instructing the I/O subsystem to sendthe subsequent command message containing a non-sequential commandfollowing the one or more commands in the received command message. 6.The computer program product of claim 1 wherein the method furthercomprises: determining that one or more commands associated with acommunication exchange cannot execute; sending a termination statusmessage to the I/O subsystem indicating an inability to execute; andclosing open communication exchanges with sequence numbers greater thana sequence number associated with the one or more non-executablecommands.
 7. The computer program product of claim 1 wherein the I/Osubsystem is a channel subsystem, and the received command message is atransport command information unit comprising a transport commandcontrol block (TCCB) with the one or more commands in one or more devicecontrol words (DCWs).
 8. An apparatus for processing a transport controlchannel program with chain linked branching at a control unit in an I/Oprocessing system, the apparatus comprising: a control unit configuredfor communication with an I/O subsystem of the I/O processing system,the control unit configured to perform a method comprising: receiving alogin request message; transmitting a login accept message in responseto the login request message, the login accept message including amaximum linked commands field indicating a maximum count of additionalcommand messages after a first command message that the control unitsupports; receiving a command message at the control unit from the I/Osubsystem to perform an I/O operation, wherein the received commandmessage is one of the first command message and the additional commandmessages; reading a chain linked flag in the command message, the chainlinked flag indicating that a subsequent command message for the I/Ooperation follows the received command message; reading a serializationflag in the received command message, the serialization flag requestingthat device status be returned to the I/O subsystem in order to selectthe subsequent command message; executing one or more commands in thereceived command message; and transmitting the device status to the I/Osubsystem in response to executing the one or more commands incombination with the serialization flag.
 9. The apparatus of claim 8wherein the method further comprises: determining that the subsequentcommand message is expected in response to the chain linked flag; andcontinuing to run counters associated with the I/O operation to span thereceived command message and the subsequent command message.
 10. Theapparatus of claim 9 wherein the method further comprises: receiving thesubsequent command message including a subsequent set of one or morecommands; examining command chain flags associated with each command inthe subsequent set of one or more commands received to locate a finalchain linked command; and transmitting a response message with extendedstatus for the I/O operation in response to locating and executing thefinal chain linked command.
 11. The apparatus of claim 8 wherein thedevice status includes a channel end (CE) and a device end (DE)instructing the I/O subsystem to send the subsequent command messagecontaining a sequential command following the one or more commands inthe received command message.
 12. The apparatus of claim 8 wherein thedevice status includes a channel end (CE), a device end (DE), and astatus modifier (SM) instructing the I/O subsystem to send thesubsequent command message containing a non-sequential command followingthe one or more commands in the received command message.
 13. Theapparatus of claim 8 wherein the method further comprises: determiningthat one or more commands associated with a communication exchangecannot execute; sending a termination status message to the I/Osubsystem indicating an inability to execute; and closing opencommunication exchanges with sequence numbers greater than a sequencenumber associated with the one or more non-executable commands.
 14. Theapparatus of claim 8 wherein the I/O subsystem is a channel subsystem,and the received command message is a transport command information unitcomprising a transport command control block (TCCB) with the one or morecommands in one or more device control words (DCWs).
 15. A method forprocessing a transport control channel program with chain linkedbranching at a control unit configured for communication with aninput/output (I/O) subsystem in an I/O processing system, the methodcomprising: receiving a command message at the control unit from the I/Osubsystem to perform an I/O operation; reading a chain linked flag inthe command message, the chain linked flag indicating that a subsequentcommand message for the I/O operation follows the received commandmessage; reading a serialization flag in the command message, theserialization flag requesting that device status be returned to the I/Osubsystem in order to select the subsequent command message; executingone or more commands in the received command message; in response todetermining that the subsequent command message is expected in responseto the chain linked flag, continuing to run counters associated with theI/O operation to span the received command message and the subsequentcommand message; and transmitting the device status to the I/O subsystemin response to executing the one or more commands in combination withthe serialization flag.
 16. The method of claim 15 further comprising:receiving the subsequent command message including a subsequent set ofone or more commands; examining command chain flags associated with eachcommand in the subsequent set of one or more commands received to locatea final chain linked command; and transmitting a response message withextended status for the I/O operation in response to locating andexecuting the final chain linked command.
 17. A computer program productfor processing a transport control channel program with chain linkedbranching at a channel subsystem configured for communication with acontrol unit in an I/O processing system, the computer program productcomprising: a tangible storage medium readable by a processing circuitand storing instructions for execution by the processing circuit forperforming a method comprising: transmitting a login request messageincluding a channel maximum linked commands field indicating a maximumcount of additional command messages queued after a command message thatthe channel subsystem supports; receiving a login accept message inresponse to the login request message, the login accept messageincluding a control unit maximum linked commands field indicating amaximum count of additional command messages queued after the commandmessage that the control unit supports; limiting transmissions to thecontrol unit to comply with the control unit maximum linked commandsfield; configuring a chain linked flag in a command message to indicatethat a subsequent command message follows a first command message toperform an I/O operation; configuring a serialization flag in the firstcommand message to request that device status be returned to the channelsubsystem in order to select the subsequent command message; andtransmitting the first command message from the channel subsystem to thecontrol unit.
 18. The computer program product of claim 17 wherein themethod further comprises: receiving a device status in response totransmitting the first command message; selecting the subsequent commandmessage in response to the received device status; and transmitting theselected subsequent command message from the channel subsystem to thecontrol unit.
 19. The computer program product of claim 17 wherein themethod further comprises: building a chain linked sequence of transportcontrol words (TCWs), wherein at least one of the TCWs includes a firstpointer identifying a first transport command control block (TCCB) toinclude in the first command message, a second pointer to a second TCWidentifying a second TCCB to include in the subsequent command messagein response to receiving a first type of device status from the controlunit, and a third pointer to a third TCW identifying a third TCCB toinclude in the subsequent command message in response to receiving asecond type of device status from the control unit.
 20. The computerprogram product of claim 19 wherein the method further comprises:linking an anchor control block (ACB) to the chain linked sequence ofTCWs, the ACB including an interrogate pointer to an interrogate TCW toaccess in response to receiving initiative to interrogate an I/O device.21. An apparatus for processing a transport control channel program withchain linked branching at a channel subsystem in an I/O processingsystem, the apparatus comprising: a channel subsystem configured forcommunication with a control unit of the I/O processing system, thechannel subsystem configured to perform a method comprising:transmitting a login request message including a channel maximum linkedcommands field indicating a maximum count of additional command messagesqueued after a command message that the channel subsystem supports;receiving a login accept message in response to the login requestmessage, the login accept message including a control unit maximumlinked commands field indicating a maximum count of additional commandmessages queued after the command message that the control unitsupports; limiting transmissions to the control unit to comply with thecontrol unit maximum linked commands field; configuring a chain linkedflag in a command message to indicate that a subsequent command messagefollows a first command message to perform an I/O operation; configuringa serialization flag in the first command message to request that devicestatus be returned to the channel subsystem in order to select thesubsequent command message; and transmitting the first command messagefrom the channel subsystem to the control unit.
 22. The apparatus ofclaim 21 wherein the method further comprises: receiving a device statusin response to transmitting the first command message; selecting thesubsequent command message in response to the received device status;and transmitting the selected subsequent command message from thechannel subsystem to the control unit.
 23. The apparatus of claim 21wherein the method further comprises: building a chain linked sequenceof transport control words (TCWs), wherein at least one of the TCWsincludes a first pointer identifying a first transport command controlblock (TCCB) to include in the first command message, a second pointerto a second TCW identifying a second TCCB to include in the subsequentcommand message in response to receiving a first type of device statusfrom the control unit, and a third pointer to a third TCW identifying athird TCCB to include in the subsequent command message in response toreceiving a second type of device status from the control unit.
 24. Theapparatus of claim 23 wherein the method further comprises: linking ananchor control block (ACB) to the chain linked sequence of TCWs, the ACBincluding an interrogate pointer to an interrogate TCW to access inresponse to receiving initiative to interrogate an I/O device.
 25. Amethod of processing a transport control channel program with chainlinked branching at a channel subsystem configured for communicationwith a control unit of an I/O processing system, the method comprising:transmitting a login request message including a channel maximum linkedcommands field indicating a maximum count of additional command messagesqueued after a command message that the channel subsystem supports;receiving a login accept message in response to the login requestmessage, the login accept message including a control unit maximumlinked commands field indicating a maximum count of additional commandmessages queued after the command message that the control unitsupports; limiting transmissions to the control unit to comply with thecontrol unit maximum linked commands field; configuring a chain linkedflag in a first command message to indicate that a subsequent commandmessage follows the command message to perform an I/O operation;configuring a serialization flag in the first command message to requestthat device status be returned to the channel subsystem in order toselect the subsequent command message; and transmitting the firstcommand message from the channel subsystem to the control unit.
 26. Themethod of claim 25 further comprising: receiving a device status inresponse to transmitting the first command message; selecting thesubsequent command message in response to the received device status;and transmitting the selected subsequent command message from thechannel subsystem to the control unit.
 27. The method of claim 25further comprising: building a chain linked sequence of transportcontrol words (TCWs), wherein at least one of the TCWs includes a firstpointer identifying a first transport command control block (TCCB) toinclude in the first command message, a second pointer to a second TCWidentifying a second TCCB to include in the subsequent command messagein response to receiving a first type of device status from the controlunit, and a third pointer to a third TCW identifying a third TCCB toinclude in the subsequent command message in response to receiving asecond type of device status from the control unit.
 28. The method ofclaim 27 further comprising: linking an anchor control block (ACB) tothe chain linked sequence of TCWs, the ACB including an interrogatepointer to an interrogate TCW to access in response to receivinginitiative to interrogate an I/O device.